Tag: CAVILUX HF

One of the most applied additive manufacturing method for metals is based on a laser scanner and a powder-bed where the metal powder is distributed over a tray and the processing laser melts it according to the desired pattern. After the pattern is created, a new layer of powder is added and the process repeats. The process enables accurate processing and elimination of spatter is essential. High-speed imaging with CAVILUX laser illumination provides high-quality images of the process.

Video material is courtesy of Nobby Tech Ltd. In Japan.

Send me more information

Leave us your contact information and our expert will get back to you.

Author: Stuart Laurence, Department of Aerospace Engineering, University of Maryland, College Park

1 Introduction

When a hypersonic vehicle travels through the atmosphere, a boundary layer develops in the air close to the vehicle surface. Initially (close to the nose of the vehicle) this boundary layer is laminar, but typically will transition to turbulence at some point downstream. A turbulent boundary layer produces significantly larger heat flux and frictional drag at the vehicle surface than a laminar one, so to be able to accurately predict vehicle performance, knowledge of the laminar-to-turbulent transition process is important. There are a variety of boundary-layer instabilities whose growth and breakdown can lead to transition; for slender planar or axisymmetric bodies and small incidences, a key instability mechanism is the second or Mack mode, which can be thought of physically as acoustic waves that become trapped within the boundary layer. Second-mode waves typically exhibit very high frequencies – around 100 kHz or even higher – which makes their measurement very difficult with conventional techniques. Here I describe measurements of the second-mode instability using a schlieren system incorporating a CAVILUX pulsed-diode laser.

2 Experimental configuration

Experiments were performed at two hypersonic wind tunnels: the High Enthalpy Shock Tunnel Goettingen (HEG) of the German Aerospace Center (DLR), and Hypervelocity Tunnel 9 of the Arnold Engineering Development Center (AEDC) at White Oak, Maryland. HEG is capable of reproducing the extremely high flow velocities typical of atmospheric reentry (up to 7 km/s), though for very short test periods (~1 ms). In the present experiments, the flow velocity and density were 4.4 km/s and 0.0175 kg/m3. Tunnel 9, on the other hand, can produce high Mach numbers with longer test periods (around 1 s), but with lower flow velocities. In the present Tunnel 9 experiments, a variety of flow conditions were used, all having a Mach number of approximately 14 and a flow speed of 2 km/s.

In both cases the test article was a slender, 7° half-angle cone. The flow within the boundary layer over the cone was visualized using a conventional Z-fold schlieren arrangement, as shown in figure 1.

Figure 1: Z-fold schlieren visualization set-up used in the experiments described here.

Schlieren is a technique used to visualize flow features in compressible flows: a density gradient at some location in the imaging plane within the test section (in a direction normal to the knife edge placed in front of the camera) will result in a change in intensity at the corresponding location on the image taken by the imaging device (in this case a high-speed camera). In the HEG experiments the light source was a CAVILUX Smart pulsed-diode laser and the camera was a Vision Research Phantom v1210, recording at 200 kHz. The laser was run in ultra-high-speed mode, with a repeated 4-pulse pattern as shown in figure 2.

Figure 2: Laser pulse pattern used in the HEG experiments: Δt12 is 2 μs and Δt34 is 3 μs.

This pattern was necessary because the characteristic frequency of the second-mode disturbance in this case (~600 kHz) was significantly higher than the recording frequency, so closely spaced pulse pairs were used to unambiguously resolve the wave motion (further details to be provided shortly). In the Tunnel 9 experiments, a CAVILUX HF laser providing, uniformly spaced pulses at approximately 70 kHz, was used together with a Phantom v2512 camera. The laser pulse width in the two experiments varied between 20 ns and 50 ns; such short pulse widths were necessary to freeze the high-speed flow structures in images.

3 Results

The HEG experiments were particularly challenging because of the high flow velocity (meaning high second-mode frequencies) and low density (meaning weak intensity variations in the schlieren images). An example of a visualized second-mode wave packet, visible from its oblique “rope-like” structures close to the surface, is shown as it propagates within a sequence of schlieren images in figure 3. The propagation speed of the wave packet is constant – the apparently uneven motion is a result of the laser pulse pattern. By performing two-dimensional image correlations, it is possible to recover the propagation speed – in this case it is 3.8 km/s. The unequal spacing between the two pulse pairs in figure 2 avoids problems with aliasing in these correlations. By then taking the Fourier transform of rows of pixels parallel to the cone surface, wavenumber spectra can be constructed; these can subsequently be converted into frequency spectra using the propagation speed calculated earlier.

Figure 3: Sequence of reference-subtracted schlieren images showing the propagation of a second-mode wave packet (flow is left to right)

Plots of the averaged power spectral density (PSD) at three locations downstream are shown in the left plot of figure 4. Here we see a strong peak at approximately 600 kHz – this corresponds to the second-mode frequency at these conditions. The peak grows rapidly as we move downstream, showing strong amplification of the second mode. A more detailed picture of this growth is shown in the right plot of figure 4, which is a contour plot of the PSD versus distance downstream. Further details of these measurements can be found in Laurence et al. (2016).

Figure 4: (Left) Plots of the schlieren power spectral density (PSD) near the surface at three locations downstream (s is the distance along the cone from the nose); (right) contour plot of the PSD versus distance downstream.

An example of a propagating second-mode wave packet in one of the Tunnel 9 experiments is shown in figure 5. Again we see the characteristic “rope-like” structures, though now the disturbance energy appears to be less concentrated towards the cone surface than it was in the HEG experiments.

Figure 5: Propagation of a second-mode wave packet in a Tunnel 9 experiment

In the Tunnel 9 experiments, the schlieren system was calibrated by placing a long-focal-length lens in the imaging plane and recording images of it. This enabled a calibration curve relating image intensity to the density gradient to be established. From this calibration curve, one can then quantify the growth rate of the second-mode instability. A contour plot of the integrated growth rate, or N-factor, versus distance downstream and frequency is shown in figure 6.

Again we see a strong second-mode contribution, but now at a much lower frequency of approximately 100 kHz. The decrease in this frequency moving downstream is associated with the thickening of the mean boundary layer. Such quantitative measurements are very important as they provide data against which numerical simulations and stability analysis computations can be compared. Further details of the Tunnel 9 experiments can be found in Kennedy et al. (2017).

4 Conclusions

The experiments described here demonstrate that it is possible to use high-speed schlieren techniques to perform quantitative measurements of extremely high-frequency instability waves in hypersonic boundary layers. The capabilities of CAVILUX pulsed-diode laser light sources proved instrumental in enabling these measurements.

About the author

Stuart Laurence (Ph.D) completed his graduate studies at the Graduate Aeronautical Laboratories, California Institute of Technology, in the area of hypersonic flows. He currently is Assistant Professor at the Department of Aerospace Engineering, University of Maryland, College Park

1 Description of process

The group of production technology at Ilmenau University of Technology investigates the spatter behavior in laser beam welding process. Spatter is the formation of metal droplets that leave the melt pool as the result of the flow conditions in the capillary and in the melt pool. It is known that the spatter formation depends strongly on the welding speed, but the industry requires high welding speed to increase output. The escaping droplets cause lack of material in the weld seam this leading to reduction of their mechanical properties. Furthermore, the droplets deposit on the work piece reducing the surface quality. The spatter can also deposit on the protective window of the laser optic which then needs to be replaced causing downtime that has to be avoided.

Hence, the task of the group is to understand the physical mechanisms of spattering and how it can be reduced.

The research group observes the formation of the capillary as well as the melt pool behavior around the capillary using a high-speed camera. Due to the high demands in terms of high frames rates and short shutter times, an external lighting source is needed. Here the group uses Cavitar’s CAVILUX HF illumination laser for lighting the area of interest. The reason for choosing CAVILUX HF lies in its ability to produce high qualitative and homogeneous illumination to the melt pool. The robust design of the CAVILUX system enables also easy handling. Furthermore, an integrated green laser pointer in the illumination laser unit permits a simple alignment of the focusing optic in relation to the area of interest. The involved operators welcome the comfort of easy configuration of the laser parameters and the simple synchronization of the lighting with the used Photron SAX 2 high-speed camera.

For observing the spatter behavior, the best illumination results were achieved using the transmitting light setup. Therefore, the optic of the illumination system was placed on the opposite side of the high-speed camera using the same angle of incidence like the camera.

Video 1 shows the formation of the capillary and the melt pool. Moreover, the development of a column of material on the back side of the capillary can be observed. The column increases in vertical direction with further time steps and disintegrates into several droplets. The droplets leave the melt pool resulting in the lack of material and reduction of mechanical properties of the weld.

Video 2 shows the influence of the superimposed diode laser on melt pool behavior using the same welding speed. It is clearly visible that the size of the melt pool is increased, whereby the dynamics of the melt flow is reduced. Particularly the last mentioned effect leads to a distinctive decrease of spattering.

The use of the CAVILUX illumination system in combination with the high-speed camera enables the possibility to visualize the impact of the superimposed laser spots on the weld pool behavior and hence, the formation of spatter. The observations are necessary in order to extend the knowledge of spatter formation and their reduction.

The investigations are carried out within the project ”Spatter reduction due to adapted laser intensity for high-speed welding” (01.07.2016 – 30.06.2018). The research project (IGF-18582 BR/2) is supported by the Federal Ministry of Economic Affairs and Energy within the Allianz Industrielle Forschung (AIF), which is based on a resolution of the German Parliament.

Description

The Institute of Engineering Thermodynamics at the Friedrich-Alexander University of Erlangen-Nürnberg is using CAVILUX HF laser illumination in various applications (Figure 1 – 3). The full potential of the laser lighting can be seen in shadowgraphy of fluid flows in injection nozzles. The camera, nozzle and illumination are aligned in this order at an optical rail. The laser illuminates through the transparent nozzle directly into the camera. Different phases of the liquid fuel lead to changes in the refractive index of the fluids. This can be captured by the camera. The field of view is 5 mm x 5 mm. The objective is a Navitar Long Distance Microscope. The depth of field can be adjusted with the aperture of the optics. However, this results in significant light losses. The high brightness of the CAVILUX laser illumination solves this challenge. The speed of the fluid stream in the nozzle is about 100 – 200 m/s which is captured by the camera with 200 kHz. Very short exposure times are needed to reduce motion blur. The camera does not provide short enough shutter times. Therefore the light source is the only option to reduce the motion blur. The pulsed mode of CAVILUX HF that creates short pulses at high frequencies allows to capture high quality images.

The visualization of fluid flows in the injection nozzles and the cavitation of the flows created in the nozzle would not be possible without CAVILUX HF laser illumination

Figure 1: Spray of fuel oil captured at 20.000 fps. Use of an effervescent nozzle. Field of view of 5 mm x 20 mm)

Author: Prof. Regis Henrique Goncalves e Silva, Dr. Eng.

Over the last years a profusion of new versions of arc welding processes has overwhelmed the international welding scenario in the industry and academia. Innovations have been made possible not only by means of electronics and software developments, but also through new concepts in mechanical design and mechanisms.

With respect to the TIG process, one example is Dynamic Feed (Wire Oscillation). Low productivity is often a disadvantage attributed to conventional TIG, when compared to other arc welding processes. In order to manage this drawback, as well to better deal with hard wetting materials (Ni-Cr alloys for example), a forward and backward wire oscillation movement has been implemented in TIG systems and it finds good acceptability in the industry as well as great interest within the scientific community. A further benefit of reducing porosity may also be expected from the technique. For the study and development of such techniques, high-speed filming has been a powerful tool for observation and stability evaluation of the metal fusion and transfer, arc behavior and weld pool behavior. Main objectives are scientific investigations on parameters influence over the resulting physical phenomena and development of parameterization for different welding conditions (position of wire feeding, torch geometry, wire dynamics).

Video 1:Dynamic Feed TIG Welding imaged at 1.000 fps

With respect to MIG/MAG welding, new technologies aim at developing adaptive control methods, innovative current waveforms and mechanization techniques in order to improve arc stability, metal transfer regularity, process reliability and expansion of the application range. Here, examples are the rotary arc and the pulsed arc mode, which are promising in achieving outstanding results for cladding and thick walled narrow gap joints. In these cases, high-speed filming is applied for metal transfer phenomena observation, arc movement patterns and respective influences over the weld pool, arc geometry and generation of the weld bead. Also, high-speed filming has been being applied to evaluation of consumables and peripherals (like wire-electrodes, contact tips and wire feeders).

In the scope of these investigations and developments, CAVILUX HF has been intensively applied. The laser illumination system allows us to finely adjust the arc intensity of the high-speed images produced, thus enabling the selection and isolation of specific welding process features (wire, arc, droplet, pool, etc.) which are goal sensitively, specifically meant to be monitored, analyzed and investigated.

Studying the flow of the protection gas of a Gas Metal Arc Welding (GMAW) process by applying welding PIV imaging method.By the Institute of Surface and Manufacturing Technology, Dresden University of Technology, Germany. Illumination is provided by CAVILUX HF with light sheet optics.

Send me more information

Leave us your contact information and our expert will get back to you.